In these years, 3GPP (Third Generation Partnership Project) completes the standardization of LTE (Long Time Evolution) that is one of high-speed data communication specifications and starts the standardization of LTE-Advanced that is an evolved system. The LTE-Advanced system targets the further increase of throughput. In other words, the LTE-Advanced system can perform communication by using a maximum bandwidth of 100 MHz when the LTE system performs communication by using a maximum system bandwidth of 20 MHz.
Moreover, the LTE-Advanced system is required to have compatibility with the LTE system, and a terminal device that employs the LTE system is required to be connected to a network of the LTE-Advanced system. To meet these demands, some consider forming one LTE-Advanced system band by bundling LTE system bands.
An example of a radio format of the LTE system is illustrated in FIG. 20. As illustrated in FIG. 20, an LTE system band 600 is divided into a control channel region 610 and a data channel region 620. The control channel region 610 includes therein a control channel 611. A terminal device transmits and receives various types of data at a predetermined resource location in the data channel region 620 on the basis of the control channel 611.
For example, when the control channel 611 is a control channel for downlink data assignment, the terminal device receives a data channel in the data channel region 620 which is located at a resource location designated by the control channel 611. On the other hand, when the control channel 611 is a control channel for uplink data assignment, the terminal device transmits data in the data channel region 620, at a resource location designated by the control channel.
Next, an example of a radio format of an LTE-Advanced system obtained by bundling three system bands of the LTE system is illustrated in FIG. 21 as an example of the LTE-Advanced system. Hereinafter, LTE system bands of the LTE-Advanced system are referred to as component carriers. As illustrated in FIG. 21, component carriers 700a to 700c respectively have control channel regions 710a to 710c and data channel regions 720a to 720c. 
In the LTE-Advanced system, a terminal device transmits and receives data on the basis of a control channel similarly to the LTE system. In this case, in the LTE-Advanced system, control channels corresponding to the component carriers are included in the control channel region of one component carrier. For example, as illustrated in FIG. 21, a control channel 730a corresponding to the component carrier 700a, a control channel 730b corresponding to the component carrier 700b, and a control channel 730c corresponding to the component carrier 700c are all included in the control channel region 710b of the component carrier 700b. 
In this way, the LTE-Advanced system previously decides one component carrier among the component carriers 700 as the one which has all the control channels 730 to be acquired by a certain terminal device. As a result, the terminal device can acquire control channels that correspond to all the component carriers 700 only by accessing the control channel region 710 of one of the component carriers 700 (see 3GPP TSG-RAN WG1#53bis, R1-082468, “Carrier aggregation in LTE-Advanced”).
When the LTE-Advanced system as described above is employed, the control channel has to carry information indicating which of the component carriers corresponds to this control channel, and therefore an amount of information included in the control channel increases. As a result, there is a possibility that an utilizable data channel region decreases and thus frequency use efficiency decreases as the whole system.